@Ineuw: Please follow https://www.mediawiki.org/wiki/Help:Locating_broken_scripts for how to use debug=true: The console of your web browser should display `Error: "Unknown dependency: mediawiki.toolbar"`.

https://en.wikisource.org/wiki/MediaWiki:Gadgets-definition defines
charinsert-core[ResourceLoader|hidden|dependencies=jquery.textSelection,mediawiki.toolbar,user]|charinsert-core.js|charinsert-core.css

This might be related to T30856: Remove classic edit toolbar from core (which was announced in Tech News).

Have attempted to load a page using the safemode param, Charinsert toolbar still fails to appear

I don't see anything obvious in the console.

But I know get - Error: "Unknown dependency: mediawiki.toolbar"
sortDependencies https://en.wikisource.org/w/load.php:1037:12
sortDependencies https://en.wikisource.org/w/load.php:1081:7
resolveStubbornly https://en.wikisource.org/w/load.php:1116:7
load https://en.wikisource.org/w/load.php:2057:17
<anonymous> https://en.wikisource.org/w/load.php:8:2
load.php:226:5

Yes, if you have a very old version of the gadget then you'll need to update it. See T30856#4713960 for how I did it for arwiki, or just copy the current version from enwiki.

This is not a breakage in ProofreadPage, but in a community-supported gadget. Generally gadgets are out-of-scope for Phabricator, and instead discussed on-wiki.

Dear Friends,
I do not have any "technical" knowledge at all, but just want to report that missing the "Charinsert" toolbar (on en.wikisource, but also on nl.wikisource) is really nasty. It would be great if this could be repaired. Many thanks in advance. --~~~~